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Miscellaneous Design Considerations

 

Table 15.11. Aircraft avionics items

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

EFIS/MFDs

Computers

Communication

Navigation

 

 

 

 

 

 

 

System display

Air data

ATC

DME

 

Analog gauges

FBW

VHF

GPS

 

Radar

FADEC

Television

DME

 

 

Autopilot

 

VHF

 

 

 

 

 

 

Civil Aircraft Application

ARINC 429 (originated in the 1970s): The success of the military standard was followed by civil standards, which began in a simplified manner. The Airbus 320 was the first large transport aircraft to use a full FBW system. ARINC 629 is the updated version that replaces ARINC 429.

Line replacement units (LRUs) are a convenient hardware design to facilitate installation and maintenance of electrical and avionics transmissions and connections following the bus standards. LRUs are constructed in the modular concept as a subassembly and then installed on an aircraft. LRUs are also standardized to comply with the bus architecture.

Aircraft communication and navigation equipment is part of an avionics package. The components of a typical civil aircraft avionics package are listed in Table 15.11.

Typical antenna locations for aircraft communication and navigation are shown in Figure 15.26. Antennas are installed in the symmetrical plane of an aircraft. Surveillance aircraft have a specific large housing for special-purpose avionics.

15.9.5 Electrical Subsystem

All aircraft must have some form of electrical supply to power the aircraft subsystems. The supply of electricity is executed by a combination of generators and batteries. Most modern aircraft require both AC and DC supplies. The typical AC voltage is 115 volts at 400 Hz, but there are higher-voltage AC supplies. Typically, the DC voltage supply is 28 volts. The electrical-supply control must ensure safety and comply with mandatory requirements.

The following systems are associated with electrical power:

engine starting and operation; management of the fuel system

lighting – both internal and external (Figure 15.27 shows external requirements)

flight deck instrumentation

communication and navigation

Figure 15.26. Antenna locations

15.9 Aircraft Systems

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Figure 15.27. Aircraft lighting requirements

avionics system

flight-control system using the PCU

passenger services for civil aircraft

APU: emergency electrical power generation and supply

armament management, electronic defensive and countermeasures for military aircraft

Typically, the electrical supply is generated at the primary and secondary levels. Engine-driven generators supply the primary power. The secondary supply serves before an engine starts and is a standby in an emergency situation. The secondary supply is generated from batteries, the APU, or an auxiliary system such as RAT.

The below-floorboard equipment bay houses items such as batteries, chargers, power controllers, transformers, and inverters (see Figure 15.27).

The weight of an electrical system depends on the load requirements. The cable weight is significant. An avionics system can be 0.4 to 4% for civil aircraft and 0.5 to 5% for military aircraft.

15.9.6 Hydraulic Subsystem

All larger aircraft have a hydraulic system, which includes a fluid reservoir, electricity-driven pumps, hydraulic lines, valves, and pilot interface at the flight deck. Hydraulic-driven actuators are incorporated at a higher force level to activate the following uses:

aircraft control system (e.g., elevator, rudder, aileron, and high-lift devices)

engine thrust reversers

undercarriage deployment and retraction

brake application

For modern civil aircraft, hydraulic pressure is from 2,000 psia (older designs) to 3,000 psia (current designs); military aircraft hydraulic oil pressure has reached 8,000 psia. A higher pressure lowers the system weight but requires stringent design considerations.

Figure 15.28 shows the hydraulics system scheme of a four-engine aircraft. To ensure safety and reliability, at least two independent, continuously operating hydraulic systems are positioned in separate locations. The port side is identified as

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Miscellaneous Design Considerations

Table 15.12. Hydraulics-driven subsystems (BAe RJ family)

Separately in both systems

Yellow system

Green system

 

 

 

Wheel brake and parking brake

Flap brake

Lift spoilers (center and outboard)

Power transfer unit (PTU)

Roll spoilers

Air brake

1 Flap mode

Lift spoilers (inboard)

Undercarriage (landing gear)

Oil reservoir

Undercarriage emergency lock

Nose-gear steering

Accumulator

1 Rudder control

Wheel brake only

 

Standby fuel pumps

Standby AC/DC generator drive

 

Internal stair

Engine-driven pump (EDP)

 

 

 

the yellow system and the starboard side is the green system. (Airbus introduced a third independent line – the blue line.) Each line has its own reservoir and functionality. Table 15.12 lists the subsystems activated by the hydraulics system. All systems include gauges, switches, valves, tubing, and connectors.

Figure 15.28. Aircraft hydraulics system

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